pxa168_eth: convert skb->end into skb_end_pointer(skb)

The change set of 4305b541, "[SK_BUFF]: Convert skb->end to sk_buff_data_t"
converted skb->end from pointer type to sk_buff_data_t.
The pointed value should be accessed via skb_end_pointer().

Since arm arch doesn't define NET_SKBUFF_DATA_USES_OFFSET,
skb->end is effectively pointer. So it doesn't cause a real problem.
But this patch is good for consistency.

Found by inspection. Compile tested only.

Cc: Simon Horman <[e-mail]>
Signed-off-by: Isaku Yamahata <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/ethernet/marvell/pxa168_eth.c

···
 		/* Get 'used' Rx descriptor */
 		used_rx_desc = pep->rx_used_desc_q;
 		p_used_rx_desc = &pep->p_rx_desc_area[used_rx_desc];
-		size = skb->end - skb->data;
+		size = skb_end_pointer(skb) - skb->data;
 		p_used_rx_desc->buf_ptr = dma_map_single(NULL,
 							 skb->data,
 							 size,